Question

If 7 times the 7th term of an AP is equal to 11 times 11th term . Find its 18th term

Answer 1

Term 7 = a + (7-1)d = a + 6d 
Term 11 = a+ (11-1)d = a+10d

According to problem,

7(a+6d) = 11(a+10d)
7a + 42d = 11a+110d
-4a = 68d
a= -17d..............................................(i)

Required to find: 18th term = a+(18-1)d = a+17d 
                                                               = -17d+17d (from (i))
                                                               = 0
Required 18th term = 0